One of the coacting tools of such a press, usually the die, is mounted on a fixed bed while the other one is vertically reciprocable above that bed by means of two hydraulic cylinders whose pistons are rigid with opposite ends of a carrier therefor. The cylinders are mounted above opposite extremities of the stationary tool on support means, generally in the form of a pair of brackets, rising from the machine bed. The admission of hydraulic fluid--referred to hereinafter as oil--into the cylinders for alternately lowering and raising the reciprocable tool carrier is governed by flow-control means which may comprise one or more solenoid valves responsive to the operation of limit switches determining the top and bottom positions of the carrier.
Despite the symmetrical positioning of the cylinders and the simultaneous application of fluid pressure to their pistons, unavoidable manufacturing tolerances frequently give rise to certain level differences between the bottom positions of the two pistons and thus of opposite ends of the tool carried thereby. These level differences will result in nonuniform penetration of the die by the blade over the length of the workpiece and may therefore lead to inadmissible variations in the cross-sectional area of the profile to be produced. Such an irregular shape may also be caused by an unevenness of the undeformed sheet material on account of which one piston, encountering a greater resistance than the other, does not descend as far as its mate.